1. Field of the Invention
The present invention relates to a method and an apparatus for correcting color mixing errors in a color display device, and more particularly to a method and an apparatus for correcting nonlinear color mixing errors in a color display device.
2. Description of the Related Art
At present, a color display device such as a cathode ray tube (CRT) displays video data based on a color mixing theory with the three primary colors: red (hereinafter referred to as R), green (hereinafter referred to as G) and blue (hereinafter referred to as B) of the color display device, and then these three are mixed with different intensities to produce various colors on the device. Taking the color CRT incorporated in the present computer system as an example, one can observe that the video signals are displayed on the color CRT on an 8-bit-per-channel scale: pure red color (R, G, B)=(255, 0, 0), pure green color (R, G, B)=(0, 255, 0), pure blue color (R. C, B)=(0, 0, 255), and white color (R, G, B)=(255, 255, 255). If the tri-stimulus values (X, Y and Z defined by the International Commission on Illumination, CIE) are used to represent the values of the measured colors, then (Xr, Yr and Zr) stands for red color; (Xg, Yg and Zg) stands for green color; and (Xb, Yb and Zb) stands for blue color. Therefore the mixed color (Xc, Yc and Zc) of a (R, G, B) signal displayed on the color CRT can be represented by the tri-stimulus values of the CIE as given in Equation (1) below:
                              [                                                                      X                  c                                                                                                      Y                  c                                                                                                      Z                  c                                                              ]                =                              [                                                                                X                    r                                                                                        X                    g                                                                                        X                    b                                                                                                                    Y                    r                                                                                        Y                    g                                                                                        Y                    b                                                                                                                    Z                    r                                                                                        Z                    g                                                                                        Z                    b                                                                        ]                    ·                      [                                                            R                                                                              G                                                                              B                                                      ]                                              (        1        )            
Then, if the defined range (such as 8 bits) of the signals for the three color R, C, and B lights are normalized as Nr=R/255, Ng=G/255 and Nb=B/255, and when the normalized values then are Nr=1, Ng=1 and Nb=1 respectively, pure red color (Xrmax, Yrmax, Zrmax), pure green color (Xgmax, Ygmax, Zgmax) and pure blue color (Xbmax, Ybmax, Zbmax) are obtained respectively.
Theoretically, the white color light is produced when Nr=Ng=Nb=1 at the same time, and its mixed color (Xw, Yw, Zw) can be represented in Equations (2) and (3) as follows:
                              [                                                                      X                  w                                                                                                      Y                  w                                                                                                      Z                  w                                                              ]                =                              [                                                                                X                                          r                      ⁢                                                                                          ⁢                      max                                                                                                            X                                          g                      ⁢                                                                                          ⁢                      max                                                                                                            X                                          b                      ⁢                                                                                          ⁢                      max                                                                                                                                        Y                                          r                      ⁢                                                                                          ⁢                      max                                                                                                            Y                                          g                      ⁢                                                                                          ⁢                      max                                                                                                            Y                                          b                      ⁢                                                                                          ⁢                      max                                                                                                                                        Z                                          r                      ⁢                                                                                          ⁢                      max                                                                                                            Z                                          g                      ⁢                                                                                          ⁢                      max                                                                                                            Z                                          b                      ⁢                                                                                          ⁢                      max                                                                                            ]                    ·                      [                                                                                N                    r                                                                                                                    N                    g                                                                                                                    N                    b                                                                        ]                                              (        2        )                                =                  [                                                                                                                Xr                      ⁢                                                                                          ⁢                      max                                        +                                                                                                                                          Yr                      ⁢                                                                                          ⁢                      max                                        +                                                                                                                                          Zr                      ⁢                                                                                          ⁢                      max                                        +                                                                        ⁢                                                                                                      Xg                      ⁢                                                                                          ⁢                      max                                        +                                                                                                                                          Yg                      ⁢                                                                                          ⁢                      max                                        +                                                                                                                                          Zg                      ⁢                                                                                          ⁢                      max                                        +                                                                        ⁢                                                                                Xb                    ⁢                                                                                  ⁢                    max                                                                                                                    Yb                    ⁢                                                                                  ⁢                    max                                                                                                                    Zb                    ⁢                                                                                  ⁢                    max                                                                                ]                                    (        3        )            
When various values of the Nr, Ng and Nb are given, different color signal values (X, Y, Z) are produced according to the color mixing model described in Equation (2). The color mixing method used to generate mixed colors is generally called a “Linear Additive Three-Color Mixing Model”.
Although most of the common cathode ray tubes adopt the aforementioned linear additive three-color mixing model, it is found that the color mixing model and the processing used in various color liquid crystal displays (LCD) today are different from that of the linear additive three-color mixing model. One of the common non-linear characteristics of LCD screens resides in the existence of a nonlinear relation between the three color control signals R, G, B and the color signals X, Y, Z. Such nonlinear relation can be represented by the nonlinear gamma characteristic as shown in FIG. (1), where the x-axis represents the control signal of the liquid crystal display and the y-axis represents the color signal. Furthermore, the liquid crystal display actually possesses additive failure characteristic, which is mainly caused by the nonlinear crosstalk characteristic of the color display device itself in such a way that there is interference produced by signal mixing, and this interference will generate interaction or crosstalk within the mixed signals X, Y and Z. For example, when a pure red color (R, G, B)=(255, 0, 0), a pure green color (R, G, B)=(0, 255, 0) and a pure blue color (R, G, B)=(0, 0, 255) are displayed separately, the sum of the values of X, Y and Z signals of the measured colors are not equal to the values of the measured X, Y and Z signals of the color when the color liquid crystal displays the control signal of a pure white color (R, G, B)=(255, 255, 255). As shown in FIG. 2, the x-axis is the control signal and the y-axis is the measured CIE Y values, whereas Y(white) represents the measured CIE Y values for pure white color ramp (by simultaneously inputting equal amounts of R, G, and B ranged from 0 to 255 into the color liquid crystal device); and Y(R)+Y(G)+Y(B) represents the sum of the individually measured CIE Y values for pure red, pure green and pure blue color ramps throughout the range. It is noteworthy that the additive failure characteristic is one of the major issues of the present color liquid crystal displays, since this characteristic causes the color mixing of the color liquid crystal display to no longer follow the linear additive color mixing model.
To overcome the nonlinear problem created by the “gamma characteristic” of a traditional color liquid crystal display, an inventor, Mr. Tae-Sung Kim of the Korean Samsung Electronics Co. Ltd., disclosed a “gamma correction circuit” in the U.S. Pat. No. 5,796,384, and such gamma correction circuit records and corrects the nonlinear relation between the three color light control signals (R, G, B) and the color display signal (X, Y, Z) of the color liquid crystal display by way of a memory device. The objective of Kim's invention is to adjust the relation between the light transmissivity and the input value of a control signal to a substantially linear manner. Even though after the gamma correction, it is possible to mathematically add the three foregoing independent linearized color lights (such as R, G and B) to produce a specific color, the actual color liquid crystal display may still have an “additive failure” phenomenon caused by the crosstalk effect such that the chromaticity of the mixed white color ramp would shift at different levels of the digital control signals. The other phenomenon is known as an “unstable primary” by which the chromaticity of the pure color ramps would shift as shown in FIG. 3. It was described in related literature such as YASHIDA 2002 (Yasuhiro Yashida and Yoichi Yamamoto, Color Calibration of LCDs, IS&T and SID The 10th Color Imaging Conference Proceedings, pp. 305-311, 2002.). Therefore, the invention of the U.S. Pat. No. 5,796,384 issued to Mr. Tae-Sung Kim simply mapped the control signal to a correction signal and intended to create a linear display between the control signal (R, G, B) and the mixed color display signal (X, Y, Z). However, Kim's invention is unable to fully solve the aforementioned problems of the color liquid crystal display.
In view of the shortcomings of the “additive failure” and the “unstable primary” problems occurred in the video color display of the prior-art liquid crystal display, the way of designing a color display so that the desired color can be displayed in a video display at a low manufacturing cost and a simple processing model becomes an important subject to the present color display designers and manufacturers.